Polymer dispersions useful as binders for coating compositions, such as high gloss trim paints, have to comply with increasingly stringent requirements. Paint manufacturers are under pressure to minimize or eliminate volatile organic compounds (VOCs) in dispersion paints due to their toxicity and flammability (Directive 2004/42/CE of the European Parliament and The Council of The European Union). To avoid the need to include coalescent agents (since these are known to be the main contributors to VOCs in coating applications), the base polymer should have a minimum film forming temperature (MFFT) equal to or less than 5° C., preferably 0° C. Additionally, styrene-based monomers have been extensively used in the production of polymeric binders since they tend to increase the gloss of the resulting coatings because of their high refractive index and since they produce polymers with good block resistance. However, concerns about the adverse health effects of styrene-based polymers have led to increased interest in styrene-free dispersions.
There is therefore a need to develop a preferably styrene-free polymer dispersion which has an MFFT equal to or less than 5° C., preferably 0° C., and which produces coatings with equivalent gloss and block resistance as existing styrene-based dispersions without the use of coalescent agents. In accordance with the invention, acrylic based polymer dispersions meeting these requirements have been produced by control of the morphology and chemistry of the polymer particles.
Gradient feed and power feed technologies have been explored as an alternative to the more traditional core-shell approach to obtain polymer dispersions with improved characteristics. In such approaches, the monomer and/or initiator compositions of the reactor feeds are varied continuously throughout the process to obtain polymer dispersions with gradually varying (gradient) morphology.
For example, U.S. Pat. No. 5,756,573 to Trumbo et al. (“Trumbo”) discloses a seed polymerized latex polymer having a gradient polymer morphology surrounding a latex seed core. The polymerization process comprises introducing latex seed particles having a number average particle size of about 20 to about 60 nanometers, and introducing a first monomer feed composition and a second monomer feed composition simultaneously to an emulsion polymerization reaction zone. The feed rates of the monomer feeds, having a Tg difference of preferably greater than 25° C., are then varied continuously to obtain a seed polymerized latex polymer having a gradient polymer morphology and a number average particle size less than about 100 nanometers. The latex polymer of Trumbo is said to be useful in the production of wood coatings having excellent print resistance and a high gloss finish. In addition to the complexity involved in preparing a latex seed particle ex situ, the latex seed particles employed in Trumbo are preferably composed of polystyrene.
U.S. Pat. No. 7,173,083 to Scheerder et al. (“Scheerder”) discloses an aqueous composition comprising components: (A) 50 to 99 wt. % of a vinyl polymer(s) having a gradient polymeric morphology; and (B) 1 to 50 wt. % of at least one polymer not having a gradient polymeric morphology, wherein components (A) and (B) add up to 100%, and wherein component (A) is blended with component (B). The monomer feeds used to prepare the polymer with a gradient polymeric morphology usually differ with respect to, for example, glass transition temperature (Tg), monomer functionality (for example the use of crosslinking, acid functional or adhesion promoting monomers), hydrophilicity, refractive index, molecular weight or simply a variation in the concentration of the respective monomers in each monomer feed. Dispersion blends usually suffer from reduced gloss, when compared to the corresponding core-shell analogs prepared in a single emulsion polymerization process. Further, Scheerder discloses styrene and derivatives thereof as a suitable vinyl monomer for forming vinyl polymer(s) with gradient polymer morphology.
U.S. Pat. No. 6,617,389 to Delaunoit et al. (“Delaunoit) discloses an aqueous polymer dispersion for use in water based glossy lacquers. The polymer dispersion is formed from monomer compositions A and B, wherein the difference of the Tg of A and B after monomer polymerization is at least 60° C. and the highest of such Tg is at least 40° C. The polymer dispersion is produced by a power feed method, wherein the monomer composition A, which is added to the reactor, is continuously replenished by monomer composition B. This power feed process is applied throughout the main polymerization phase to obtain polymer particles substantially composed of polymer chains with varying composition. In such a full gradient approach, it is difficult to achieve dispersions with a sufficiently high block resistance while exhibiting a low MFFT. Further, Delaunoit discloses styrene and derivatives thereof as suitable monomers.
U.S. Pat. No. 3,804,881 to Bassett et al. (“Bassett”) generally discloses that non-uniform copolymers can be produced by continuously introducing at least one primary polymerizable feed composition to a polymerization zone, which is continually varying in compositional content of the reactants therein, while simultaneously adding at least one different secondary polymerizable feed composition, so as to continually change the compositional content of the reactants. Bassett discloses styrene and derivatives thereof as suitable polymerizable reactants.
DE 10041680 to Porzio et al. (“Porzio”) discloses an aqueous polymer dispersion prepared by radical-initiated aqueous emulsion polymerization of monomer mixtures (M1, M2) added according to a specific feed procedure. Polymerization is performed in a vessel fed with a monomer stream (m) formed from partial streams, m1 and/or m2, of M1 and M2, respectively, and during the process the proportion of m2 in m increases. At the start of feeding, m comprises at least 90 weight percent M1 but at the end it contains at least 90 weight percent M2. When used alone, M1 produces a polymer of glass transition temp (Tg,1) not over 50° C. while M2, alone, produces a polymer with similar temperature (Tg,2) over 50° C., with at least a 10° C. difference between Tg,1 and Tg,2. The ratio of total amounts of M1 and M2 is 20:80 to 60:40. The power feed process is applied throughout the main polymerization phase. All the examples in the reference have fairly high MFFT values (>25° C.). It is further difficult to achieve dispersions with a sufficiently high block resistance while exhibiting a low MFFT by utilization of a gradient approach alone.
WO 2013/088187 to Deller et al. (“Deller”) discloses a polymer dispersion comprising particles of a polymer composition formed at least partially by emulsion polymerization of at least first and second, simultaneously added, substantially styrene-free, monomer feeds in the presence of an initiator in a reaction zone. The first monomer feed comprises monomers selected to produce a copolymer having a glass transition temperature less than or equal to about −10° C. and the second monomer feed comprises monomers selected to produce a copolymer having a glass transition temperature greater than or equal to about 50° C. The relative rate of addition of the first and second monomer feeds into the reaction zone is continuously changed during at least part of the emulsion polymerization and the rate of addition of the initiator is changed step-wise at least once during the addition of the first and second monomer feeds. Although the process of Deller yields a styrene-free polymer dispersion with a MFFT below 5° C., coatings produced from the dispersion have been found to exhibit insufficient block resistance.